Sputtered Aluminum Nitride Thin Films Research Articles

Effect of silicon-based substrates and deposition type on sputtered AlN thin films: Physical & chemical properties and suitability for piezoelectric device integration

The synergistic effect of silicon-based substrates on the physical and chemical properties of aluminum nitride (AlN) thin films was investigated. AlN thin films were deposited by RF magnetron sputtering on Low Resistivity (LR) Si, Silicon On Insulator (SOI) and Si/SiO2 substrates at room temperature. The morphological and structural properties were investigated by X-ray diffraction (XRD), Raman spectroscopy, Atomic Force microscopy (AFM). XRD analyses evidenced the co-presence of (002) and (101) orientations. The substrate influence on films morphology, crystalline order, intrinsic stress and grain size is well evidenced, as shown by Raman and AFM analyses. These surface characterization techniques represent a valid support to select the suitable Si-substrate/piezoelectric thin film combination for the fabrication of a piezoelectric device. AlN sputtered on Si-LR substrate showed an enhancement of structural arrangement along (002) planes while the sample sputtered on Si/SiO2 resulted mainly oriented along (101) planes. For these reasons, further characterization was done: (002)-oriented AlN thin films were characterized in terms of piezoelectric response by piezometer and Piezoresponse Force Microscopy (PFM) measurements, while cytotoxicity and biocompatibility were investigated for (101)-oriented AlN thin films. This further investigation helped to assess the suitable film to integrate into piezoelectric devices operating in air or in liquid, respectively.

Measurement and ab initio Investigation of Structural, Electronic, Optical, and Mechanical Properties of Sputtered Aluminum Nitride Thin Films

Aluminum Nitrides (AlN) thin films have been deposited ‎on glass substrates using DC-magnetron sputtering technique for different back pressure. The piezoelectric and related properties of highly c-axis oriented AlN films fabricated by dc planar magnetron sputtering have been calculated. Experimental results show that highly c-axis oriented AlN films can be fabricated by dc planar magnetron sputtering. X-ray powder diffraction (XRD) technique shows that AlN thin films exhibit a hexagonal structure. Vienna Ab initio Simulation Package (VASP) within the framework of density functional theory (DFT) and generalized gradient approximation (GGA) was used to investigate the structural and electronic properties of hexagonal AlN structures. The experimental lattice parameters of the as prepared thin films are found to be in good agreement with ab-initio calculated parameters. UV–Vis spectrophotometer measurements are performed to investigate the optical properties of AlN thin films. We found that the refractive index of AlN thin films exhibits values ranging between 2.1 and 2.2. Furthermore, the elastic, piezoelectric and dielectric tensors of AlN crystal are calculated using VASP. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of AlN. The value of the principle component of electronic contribution to the static dielectric tensor of AlN is found to be ≈ 4.68 that is in good agreement with the experimental static dielectric constant. In addition, clamped-ion piezoelectric tensor is calculated. The diagonal components of the piezoelectric tensor are found to be e_33=1.784 C/m^2 and e_31=-0.8 C/m^2. The large values of the piezoelectric coefficients show that polar AlN crystal exhibits a strong microwave piezoelectric effect. Furthermore, the components of the elastic moduli tensor are calculated. The extraordinary electronic, optical, piezoelectric and elastic properties make AlN thin films potential candidates for several opto-electronic, elastic, dielectric and piezoelectric applications.

A systematic review of magnetron sputtering of AlN thin films for extreme condition sensing

In this article, a systematic overview of the published works on magnetron sputtering of AlN thin films for harsh condition applications is presented. Harsh environments may be defined as the operating conditions exhibiting extreme shock loads, high temperature, high pressure/forces and corrosive media. Some of the harsh environments include petrochemical plants, automobile/engine, marine, aerospace, in-situ health monitoring, prosthetics, etc. In these applications, sensors are necessary to monitor and control process parameters such as pressure, temperature, fluid flow, chemical contents, PH and among others. As such, the sensor materials should be able to withstand the high and cyclic conditions within such operations. Aluminum nitride (AlN) is a ceramic material, which finds application in micro-electromechanical systems (MEMS) such as acoustic sensors, energy harvesters, transducers and resonators. These applications are motivated by its (AlN) high piezoelectric effect, high surface acoustic wave velocity, excellent dielectric permittivity, high thermal stability, wide band gap, chemical inertness, and so forth. As such, AlN films have been deposited on various non-metallic substrates such as Si, sapphire and polymers and on metallic substrates such as Ti6Al4V, diamond, Al, Cr and 304L stainless steel for different sensing applications. It is deduced that studies involving characterization of RF magnetron sputtered AlN thin films in harsh conditions are still scarce. The review serves as a resource for researchers and industry for development of sensors and transducers for harsh condition applications.

Optical characterization of sputtered aluminum nitride thin films – correlating refractive index with degree of c-axis orientation

Aluminum Nitride (AlN) is a well-known compound piezoelectric material with high Complementary Metal-Oxide Semiconductor process compatibility. Previous results show that the piezoelectric coefficient correlates with the c-axis orientation of AlN. In this work, the optical properties of reactively sputtered AlN thin-films have been investigated to find a relation with the structural properties of the film (i.e. c-axis orientation) using spectroscopic ellipsometry. The results show that for almost the same film thickness, highly c-axis orientated films have higher refractive indices compared to films with low c-axis orientation or amorphous layers. A relationship between (002) peak intensity measured with x-ray diffractometry and refractive index is shown. At 546 nm wavelength, the refractive index decreased from 2.15 for films with high (002) peak intensity, to 2.11 for films with about half the peak intensity, and further down to 1.90 for films without preferred orientation. Additionally, with the same sputtering conditions, the variation of the film thickness seems to have no significant effect on the refractive index. The results of AlN thin-film mass density obtained from x-ray reflectivity measurements are consistent with refractive index measurements. The mass density of AlN thin-films with high c-axis orientation that resulted in a higher refractive index is 2.99 g/cm−3, compared to 3.23 g/cm−3 for epitaxial grown AlN layers. This value decreased to 2.82 g/cm−3 for films with poor c-axis orientation.

Growth assessment and scrutinize dielectric reliability of c-axis oriented insulating AlN thin films in MIM structures for microelectronics applications

The present study reports the effect of bottom electrodes (Al, Pt & Ti) on the texture, piezoelectric characteristics, dielectric properties and leakage current behavior of reactive DC magnetron sputtered AlN thin films. X-ray diffraction results revealed that the lattice structure and texture of bottom electrode plays a vital role in c-axis wurtzite phase growth of aluminum nitride (AlN) thin films. A highly (002) oriented c-axis AlN thin films was obtained over Ti as compared to Al and Pt electrodes. The piezoelectric coefficient (d33eff) of these fabricated AlN thin films were measured by piezoresponse force microscopy to evaluate their piezoelectric efficiency in resonators. AlN is a good option for passivation purpose in microelectronic devices because of high chemical stability and excellent dielectric properties. Therefore, the dielectric reliability of AlN thin films is very crucial. Among all structures, AlN/Ti structure exhibits a highest dielectric constant (εr) ∼ 8.63 at 1 MHz. This work also provided a comprehensive study of leakage current conduction mechanism with electric field. The leakage behavior depicts that all AlN film endures a transition from ohmic conduction (E < 70 kV/cm) to SCLC conduction (E > 70 kV/cm). In addition, the breakdown characteristic of these fabricated films was studied for deciding the long-term device reliability and life. This study suggests that the electrode engineering of AlN thin films have a potential in devices for electrical and micro-electro-mechanical system (MEMS) applications.

Anisotropic piezoelectric response of ion beam sputtered aluminum nitride thin films textured along [10[formula omitted]0]-axis: A field dependent X-ray diffraction investigation

a-axis oriented AlN thin films sandwiched between Ti-electrodes were prepared on Si (100) substrates using ion beam sputtering in reactive assistance of N2-plasma. Piezoelectric response of such Ti/AlN/Ti heterostructures was investigated using XRD under applied voltage varying from +1 V to +5 V. External electric field induced strain in film microstructure was used to estimate the longitudinal piezoelectric coefficient d33eff. Our results show that AlN thin films with a preferred orientation along a-axis possess enhanced value of d33eff = 560.5±68 pm/V. A phenomenological model is proposed to account this enhancement by considering the dielectric an elastic continuum with Al+— N−— Al+ quadruples as the constituent unit of the Cosserat medium.

Tailored wafer holder for a reliable deposition of sputtered aluminium nitride thin films at low temperatures

Active actuated resonant micro-electro-mechanical-systems (MEMS) are used for sensing purpose like topography analysis and viscosity sensors. Those applications require straight beams and they rely on controlled film stress of the involved thin films, e.g. the active piezoelectric aluminium nitride (AlN) layer. The AlN consists of aluminium and nitrogen and is deposited with a reactive sputter process. The deposition process heats up the substrate and therefore the wafer bow of the substrate causes a variation of the thermal connection between wafer and sample holder. This goes along with undefined film stress of the AlN layer. In order to minimize the derivation of film stress, the reduction of substrate temperature and the enhancement of thermal connection between substrate and substrate holder is targeted. Therefore a novel clamped substrate holder is designed. High thermal connection to the ambient equipment, equal heat distribution and clamping of wafer stabilize the deposited AlN layer. By examining the layer stress and applying an acid structuring method, an improvement of deposited film is observed. A long term study with AlN deposition with thicknesses of 0.5µm, 1.0µm and 2.0µm on silicon wafers was made to confirm the enhancement.

Seed layer effect on the (0 0 2) texture and piezoelectric properties of pulsed-DC sputtered AlN thin films

Piezoelectric aluminium nitride (AlN) thin films were deposited by reactive sputtering of Al metal target in nitrogen atmosphere using a pulsed closed field unbalanced magnetron sputtering system on various substrates using different underlayers (seed layer) such as Pt and Mo. The texture, orientation and piezoelectric properties of AlN films were characterised by means of X-ray diffraction, field emission scanning electron microscopy and laser interferometry. A Michelson laser interferometer was designed to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that the incorporation of different seed layers at various working pressure in the sandwiched structure, significantly affected the (0 0 2) orientation and the piezoelectric response of AlN thin films.

Improved piezoelectric constants of sputtered aluminium nitride thin films by pre-conditioning of the silicon surface

The group III–V material aluminium nitride (AlN) is frequently used in micro-electromechanical devices and systems (MEMS) due to its piezoelectric properties, its high thermal and electrical stability as well as its compatibility with CMOS technology. But, the trend towards miniaturization of MEMS devices requests a continuous decrease in geometrical dimensions of the active AlN thin film, thus demanding at least the same piezoelectric properties at lower film thickness. In this work, two different approaches are applied to measure the piezoelectric coefficients, using the direct as well as the converse piezoelectric effect. The first approach utilizes laser doppler vibrometry measurements in combination with finite element analysis, allowing the determination of d33 and d31. For the second method, an oscillating force is applied to the thin film and the generated charge is measured. A surface-near substrate conditioning step applying sputter etching is used in order to improve the piezoelectric coefficients over a wide thickness range (i.e. 40 nm to 400 nm) by about 20% compared to samples without pre-treatment. Basically, the coefficients remain constant for a film thickness of 100 nm and above, thus allowing the application of thin active layers of aluminium nitride without any reduction in the sensing and actuation potential.

Impact of film thickness and temperature on the dielectric breakdown behavior of sputtered aluminum nitride thin films

In micromachined devices, piezoelectric aluminum nitride (AlN) thin films are commonly used as functional material for sensing and actuating purposes. Additionally, AlN features a high chemical and thermal stability, making it also a good choice for passivation purposes for microelectronic devices. With those aspects and current trends towards minimization in mind, the dielectric reliability of ultra-thin AlN films is of utmost importance for the realization of advanced device concepts.In this study, we present a systematic analysis of the transversal dielectric strength of sputtered AlN thin films of varying film thickness (i.e. 70nm to 400nm) up to 300°C. Using a time-zero approach, the dielectric strength is measured by applying a voltage ramp to stress the film up to the point of breakdown. The results are subsequently analyzed using the Weibull approach.For temperatures T>180°C, the characteristic breakdown field Eb,0 follows the relationship Eb,0∼T2, while showing a weak temperature dependence below, thus indicating a transitory regime between thermally induced breakdown at higher T and electrically induced breakdown at lower T. It is shown, that the characteristic breakdown field increases at lower film thickness which is in good correlation with a corresponding decrease in leakage current.

Impact of annealing temperature on the mechanical and electrical properties of sputtered aluminum nitride thin films

Aluminium nitride (AlN) is a promising material for challenging sensor applications such as process monitoring in harsh environments (e.g., turbine exhaust), due to its piezoelectric properties, its high temperature stability and good thermal match to silicon. Basically, the operational temperature of piezoelectric materials is limited by the increase of the leakage current as well as by enhanced diffusion effects in the material at elevated temperatures. This work focuses on the characterization of aluminum nitride thin films after post deposition annealings up to temperatures of 1000 °C in harsh environments. For this purpose, thin film samples were temperature loaded for 2 h in pure nitrogen and oxygen gas atmospheres and characterized with respect to the film stress and the leakage current behaviour. The X-ray diffraction results show that AlN thin films are chemically stable in oxygen atmospheres for 2 h at annealing temperatures of up to 900 °C. At 1000 °C, a 100 nm thick AlN layer oxidizes completely. For nitrogen, the layer is stable up to 1000 °C. The activation energy of the samples was determined from leakage current measurements at different sample temperatures, in the range between 25 and 300 °C. Up to an annealing temperature of 700 °C, the leakage current in the thin film is dominated by Poole-Frenkel behavior, while at higher annealing temperatures, a mixture of different leakage current mechanisms is observed.

Impact of film thickness on the temperature-activated leakage current behavior of sputtered aluminum nitride thin films

Aluminum nitride (AlN) is a material of significant importance in the field of micro electro-mechanical systems (MEMS) due to its piezoelectric properties and its use as passivation layer. This work provides a comprehensive study on the film thickness dependence of the temperature activated leakage current behavior of sputtered aluminum nitride thin films. The thickness ranges from 40 to 400nm, providing insight into the electrical characteristic of AlN thin films for thickness values typically used in MEMS devices. The leakage current shows a highly symmetrical behavior for both positive and negative bias directions due to a tailored silicon substrate pre-treatment process. At low electric field strengths E≤0.1MV/cm, the leakage current is dominated by ohmic behavior, while for E≥0.3MV/cm, the leakage current is controlled by a Poole–Frenkel mechanism. Both the defect-related barrier height and the defect density for the Poole–Frenkel regime can be extracted from arranging the data in an Arrhenius configuration. The barrier height shows no significant influence of film thickness. However, the defect density correlates directly with the leakage current level of the film and thus dominates the electrical behavior. The defect density increases significantly with increasing film thickness. As the thin films were deposited under nominally unheated substrate conditions the substrate temperature increases with increasing sputter time. A simple model is proposed in order to provide a qualitative understanding of the observed effect by assuming a substrate temperature driven redistribution of defect states within the band gap of aluminum nitride thin films.

Thickness dependence of Young's modulus and residual stress of sputtered aluminum nitride thin films

Aluminum nitride thin films are commonly used as active layer in micro-/nanomachined devices due to their piezoelectric properties. In order to predict the performance of advanced device architectures, careful modelling and simulation using techniques such as finite element analysis are of the utmost importance. An accurate knowledge of the corresponding thin film material properties is therefore required. This work focuses on the mechanical properties residual stress and Young's modulus over a wide thickness range from 100 to 1200 nm. The load-deflection technique is used to measure the bending curve of a circumferentially clamped, circular aluminum nitride diaphragm under a uniformly distributed pressure load. The bending curves are analyzed using an advanced analytical approach rather than commonly used models for load-deflection methods, thus resulting in a higher accuracy. It is found that the Young's modulus is nearly independent of film thickness, whereas the tensile residual stress exhibits a maximum at a thickness of about 600 nm. A thorough discussion of possible error sources is presented and approaches to minimize their impact are discussed.

Thickness dependent residual stress in sputtered AlN thin films

Thin aluminum nitride (AlN) films of different thickness are deposited by DC-pulsed magnetron sputtering under identical conditions on sapphire (0001) and silicon (100) substrates. An investigation of the residual stress, morphology and structural properties is carried out. The thickness of the films covers the range from 17nm to 3.9μm. A higher compressive residual stress is measured for the thinner films and the presence of a stress gradient is proven. X-ray diffraction (XRD) studies show that all AlN films are achieved with perfect c-axis orientation perpendicular to the film surface and that the films are biaxially strained. XRD rocking curves reveal that AlN films on sapphire are highly oriented for all film thicknesses, whereas AlN film growth on silicon starts highly disoriented and the film quality improves with film thickness. Surface analysis by atomic force microscopy shows a continuous film roughening and decrease of grain boundary density with increasing film thickness.

Impact of magnetron configuration on plasma and film properties of sputtered aluminum nitride thin films

We have investigated the growth of the c-axis oriented aluminum nitride (AlN) thin films on (100) silicon by reactive dc magnetron sputtering at low temperature, considering the effect of the magnet configuration on plasma and film properties. It appears that a magnet modification can significantly modify both the plasma characteristics and the film properties. Electrical and optical characterizations of the plasma phase highlight that depending on the magnet configuration, two very different types of deposition process can be involved in the same deposition chamber. On the one hand, with a balanced magnetron (type 1), the deposition process enhances the production of AlN dimers in the plasma phase and enables to synthesize AlN films with different preferential orientations (100, 002, and even 101). On the other hand, a strongly unbalanced magnetron (type 2) provides a limited production of AlN species in the plasma phase and a strong increase in the ratio of ions to metal atom flux on the growing films. In the latter case, the ion energy provided by the ion flux to the growing film is typically in the 20–30 eV range. Thus, dense (002) oriented films with high crystalline quality are obtained without any substrate heating.

Deposition and characterization of reactive magnetron sputtered aluminum nitride thin films for film bulk acoustic wave resonator

This work studies the relationship between the deposition process parameters and the properties of sputtered c-axis-oriented aluminum nitride (AlN) thin films. AlN films were deposited on a Pt electrode by reactive magnetron sputtering under various deposition conditions. The films were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). A polycrystalline AlN film with highly c-axis-preferred orientation was achieved. The XRD rocking curve was 2.7°. The FESEM photographs also show that the AlN film has a dense hexagonal surface texture with uniform grain size and a highly ordered column structure.

Large Pyroelectric Response from Reactively Sputtered Aluminum Nitride Thin Films

We report the pyroelectric response of c-axis oriented, undoped, wurtzite, aluminum nitride reactively sputtered onto polished silicon wafers. The voltage between a metallic contact on the AlN surface and the n + -doped silicon substrate was monitored during pulsed infrared, radiant heating. From analysis of the data, a pyroelectric voltage coefficient, P v , in excess of 0.5 × 10 6 V/m/K was extracted for films in the 600 to 2500 A thickness range.

Pulsed photothermal reflectance measurement of the thermal conductivity of sputtered aluminum nitride thin films

We report on measurements of the thermal conductivity of reactively sputtered aluminum nitride (AlN) thin films with different thickness, ranging from 100nm to 1μm, on silicon substrates. The measurements were made at room temperature using the pulsed photothermal reflectance technique. The thermal conductivities of the sample are found to be significantly lower than the single-crystal bulk AlN and increase with an increasing thickness. The thermal resistance at the interface between the AlN film and the silicon substrate is found to be about 7–8×10−8m2K∕W.

Piezoelectric properties and residual stress of sputtered AlN thin films for MEMS applications

In this work we investigate the suitability of piezoelectric aluminium nitride (AlN) films deposited by RF sputtering as the actuating element in microelectromechanical system (MEMS) devices. We have studied the influence of some sputtering parameters (substrate bias voltage and pressure) on the preferred orientation, grain size, residual stress and piezoelectric response of the films. X-ray diffractometry (XRD) has been used to obtain the preferred orientation and grain size. The residual stress was deduced from measurements of wafer curvature. The piezoelectric response was evaluated from the frequency response of surface acoustic wave (SAW) filters. A careful control of the energy supplied to the substrates enabled us to grow films with pure c-axis orientation and good piezoelectric response. The sputtering pressure had to be adjusted to reduce the amount of residual stress. As a result, we have determined the relationship between the sputtering parameters and the film characteristics. We present preliminary results of the fabrication of micromachined suspended bridge structures formed by AlN/polysilicon bimorphs.

Piezoelectric properties and residual stress of sputtered AlN thin films for MEMS applications

In this work we investigate the suitability of piezoelectric aluminium nitride (AlN) films deposited by RF sputtering as the actuating element in microelectromechanical system (MEMS) devices. We have studied the influence of some sputtering parameters (substrate bias voltage and pressure) on the preferred orientation, grain size, residual stress and piezoelectric response of the films. X-ray diffractometry (XRD) has been used to obtain the preferred orientation and grain size. The residual stress was deduced from measurements of wafer curvature. The piezoelectric response was evaluated from the frequency response of surface acoustic wave (SAW) filters. A careful control of the energy supplied to the substrates enabled us to grow films with pure c -axis orientation and good piezoelectric response. The sputtering pressure had to be adjusted to reduce the amount of residual stress. As a result, we have determined the relationship between the sputtering parameters and the film characteristics. We present preliminary results of the fabrication of micromachined suspended bridge structures formed by AlN/polysilicon bimorphs.